Howdy! Here’s a link to an older feature request about this: Beta & Release Candidates Tags · Issue #20 · roots/wordpress · GitHub
We’re in the process of moving to a new packager for the roots/wordpress Composer package. See RFC: roots/wordpress package switching the "no-content" WordPress release · Issue #32 · roots/wordpress · GitHub and https://github.com/roots/wordpress-packager
You can give the new roots/wordpress-full package a shot: roots/wordpress-full - Packagist
It currently has 6.0-RC1 published